1. Field of the Invention
The present invention relates to a malfunction detection method in a die clamping step in an injection molding machine.
2. Description of the Related Art
In an injection molding machine, after a formed product is pushed out of an associated die, dies are closed and clamped, then an injection step is started. For example, Jpn. Pat. Appln. KOKAI Publication Nos. 09-085792 and 2001-030326 discloses a malfunction detection in the die clamping step.
The following is the method described in Jpn. Pat. Appln. KOKAI Publication No. 09-085792. That is, when die clamping is normally carried out, a relation between a torque of a motor for driving a movable platen and a position at that time is taken, and it is recorded as a reference pattern. The monitoring area is divided with respect to the position of the movable platen into a plurality of sections, and an tolerance range of the torque relative to the reference pattern is preset at each section. At the subsequent die clamping step, the variation of the torque with respect to the position is monitored and, if the torque value falls outside the tolerance range for the corresponding section, it is decided that a malfunction occurs and an alarm is issued.
On the other hand, the method described in Jpn. Pat. Appln. KOKAI Publication No. 2001-030326 is as follows. That is, a ‘disturbance estimation observer’ is provided for estimating a load on a servomotor for driving the die clamping mechanism. The disturbance torque is estimated by the disturbance estimation observer for each die clamping. When the estimated disturbance torque is greater than a later-described upper limit value, an alarm is issued, in which case the upper limit value is obtained from an average of those estimated disturbance torques taken at a plurality of preceding die clamping steps.
In the former system, the torque of the motor for the driving the movable platen with respective to the position is monitored in the die clamping step. Since the tolerance range of the torque has its upper and lower limit values set to be constant for each section, unless the torque falls outside the allowable range in the section, the die clamping step is decided as being normally finished and shifting is made to the next injection step. In this case, a variation rarely occurs in such a way that, in the die clamping operation, up/down vibrations are produced with a given drive motor torque value as a center. Even if, therefore, the monitoring area is divided into a plurality of sections, a complicated variation occurs in respective sections. It is, therefore, necessary that the allowable load range be set to be somewhat broader in the respective section. If, however, there occurs a malfunction, it is not possible to quickly stop the motor.
On the other hand, in the latter system, if the disturbance estimation observer to be able to function properly, any disturbance torque must be sampled for each period and its value must be stored in memory in a control apparatus. It is, therefore, necessary to store its value not only for one die clamping step but also at a plurality of die clamping steps. Thus, it involves a very large amount of data and prevents effective use of a restricted capacity of memory.
In order to, in the injection molding, accurately fit together a pair of dies and, by doing so, clamp together the dies, as shown in FIG. 6, a guide pin 74 is mounted at one (movable die 72) of these dies and a corresponding guide bush 73 is embedded in the other die (stationary die 71) to allow the guide pin 74 to be fitted therein. In the case where, as shown in FIG. 7, use is made of dies having an undercut 88, angular pins and slide core 85 are used for swifter removal of a formed product 89. Here, the angular pin 83 is mounted to the fixed die 81 by use of a locking block 84 and the slide core 85 is attached to the movable die 82 by means of a coil spring 86 for core retraction. In this connection it is to be noted that a movable range of the slide core 85 is restricted by a stop block 87.
In the case where, during the die clamping step using such an angular pin, a pair of new dies are first used for die clamping, there is sometimes the case where, as shown in FIG. 7, the pin will not smoothly fit in the bush. Even where, in such a case, the die clamping force becomes somewhat high, if the operation is continued as it is, it is possible to normally finish the die clamping operation. However, in the above-mentioned conventional malfunction monitoring method, there is a higher possibility that the operation will be decided as being in an abnormal state, resulting in the die clamping operation being stopped.